Recently, as a next generation high-speed nonvolatile random access memory (NVRAM) to replace a flash memory, various kinds of device structures such as a FeRAM (Ferroelectric RAM), a MRAM (Magnetic RAM), a PRAM (Phase Change RAM) are proposed and there are severe development races among them, seeking for high performance, high reliability, low cost and process consistency. However, the above memory devices at the present have both merits and demerits and an ideal “universal memory” having all merits of a SRAM, a DRAM, and a flash memory is far from practical use.
A nonvolatile RRAM (Resistive Random Access Memory) using a variable resistance element whose electric resistance is reversibly changed by applying a voltage pulse has been proposed based on the existing technique. This constitution is shown in FIG. 1.
As shown in FIG. 1, a conventional variable resistance element comprises a lower electrode 3, a variable resistor 2 and an upper electrode 1 laminated sequentially in which its resistance value can be reversibly changed when a voltage pulse is applied between the upper electrode 1 and the lower electrode 3. A new nonvolatile semiconductor memory device can be implemented by reading the resistance value changed by the reversible resistance changing operation (referred to as “switching operation” hereinafter).
The nonvolatile semiconductor memory device comprises a memory cell array having a plurality of memory cells each having a variable resistance element and disposed in a row direction and column direction like a matrix, and peripheral circuits for controlling programming, erasing and reading actions of data for each memory cell of the memory cell array. Thus, this memory cell includes memory cells having different components, such as a memory cell having one selection transistor T and one variable resistance element R (called “1T/1R type”) and a memory cell having only one variable resistance element R (called “1R type”).
Meanwhile, regarding a material constituting the variable resistor 2, a method for changing an electric resistance reversibly by applying a voltage pulse to a perovskite material known for having a colossal magnetoresistance effect is disclosed by Shangquing Liu, Alex Ignatiev et al., in University of Houston, U.S.A. in the following patent document 1 and a non-patent document 1. This is an extremely epoch-making method because even when the perovskite material known for having the colossal magnetoresistance effect is used, a resistance change ranging over several digits can be seen at room temperature without applying a magnetic field. In addition, according to an element structure shown in the patent document 1, the variable resistor is formed of a perovskite-type oxide such as a crystalline praseodymium calcium manganese oxide Pr1-XCaXMnO3(PCMO) film.
In addition, it is known from a non-patent document 2 and a patent document 2 that regarding the material of the variable resistor 2, a titanium oxide (TiO2) film, a nickel oxide (NiO) film, a zinc oxide (ZnO) film, and a niobium oxide (Nb2O5) film formed of a transition metal oxide also show reversible resistance change. Especially, titanium oxide and nickel oxide is considered to be a material in which the resistance is changed when a region in which resistivity is locally lowered in the oxide (referred to as “filament path” occasionally hereinafter) is formed and the filament path is broken due to heat increase caused by a current flowing in the variable resistance element.
Furthermore, it is known from the non-patent document 2 and the patent document 2 that regarding the material of the variable resistor 2, the titanium oxide (TiO2) film, the nickel oxide (NiO) film, the zinc oxide (ZnO) film, and the niobium oxide (Nb2O5) film formed of the transition metal oxide also show reversible resistance change. Among the above materials, the phenomena of switching operations with titanium oxide and nickel oxide are reported in detail in non-patent documents 3 to 6 and a non-patent document 7, respectively.    Patent document 1: U.S. Pat. No. 6,204,139    Non-patent document 1: Liu, S. Q. et al., “Electric-pulse-induced reversible Resistance change effect in magnetoresistive films”, Applied Physics Letter, Vol. 76, pp. 2749-2751, in 2000    Non-patent document 2: H. Pagnia et al., “Bistable Switching in Electroformed Metal-Insulator-Metal Devices”, Phys. Stat. Sol. (a), vol. 108, pp. 11-65, in 1988    Patent document 2: Japanese Unexamined Publication of PCT Application No. 2002-537627    Non-patent document 3: G. Taylor et al., “RF Relaxation Oscillations in Polycrystalline TiO2 Thin Films”, Solid-State Electronics, 1976, vol. 19, pp. 669-674    Non-patent document 4: F. Argall et al., “Switching Phenomena in Titanium Oxide Thin Films”, Solid-State Electronics, Pergamon Press 1968, vol. 11, pp. 535-541    Non-patent document 5: Beam et al., Proc. IEEE, 52, 300-1, 1964    Non-patent document 6: F. Argall, Solid State Electronicis Pergamon Press 1968, vol. 11, pp. 535    Non-patent document 7: S. Seo et al., Applied Physics Letters 86, 093509, 2005
When the perovskite-type oxide is used as the material of the variable resistor 2 whose resistance is changed in response to the voltage pulse, since its crystallization temperature is as high as 500° C. to 700° C., it cannot be formed after the wiring of an LSI is formed. In addition, most constituent elements of perovskite are not used in a LSI process and since these constituent elements could affect device characteristics, it is necessary to examine and take measures against contamination of these elements.
Meanwhile, when the variable resistor 2 is formed of titanium oxide or nickel oxide, since titanium and nickel is largely used in the LSI process, they do not affect the device characteristics. However, the resistance change of the variable resistance element formed of titanium oxide and nickel oxide conventionally considered is based on a phenomenon in which low resistance and high resistance are provided when a filament path is formed and broken depending on a voltage pulse applying condition. Thus, in order to provide the switching operation, it is necessary to form the filament path by applying a specific voltage first (referred to as “forming process” hereinafter).
In addition, according to the above element, the problems are that the diameter of the filament path is increased as the number of switching operations is increased, that the resistance value fluctuates due to the change of filament density, and that it is difficult to control the resistance value because there is no area dependency in the element in a low resistance state since the resistance value is determined by a filament, so that it is not practically used as a device at the present.
The present invention was made in view of the above problems and it is an object of the present invention to provide a variable resistance element superior in LSI process consistency, capable of performing a resistance switching operation without a filament path, and showing a stable resistance value retention characteristics.